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Double-ridged waveguide can provide more bandwidth than "normal" rectangular waveguide. Double-ridged waveguide is similar to finline.
A pair of ridges protrude into the center of the waveguide, parallel to the short wall. This is where the E-field is maximum. By bringing ground down the ridges, the E-field is further increased.
That was a lame explanation... while we're waiting for more content on double-ridged waveguide, let's step back and look at its cousin.
Single-ridged waveguide
Here's a discussion of single-ridged waveguide, from Mohammed. Thanks!
Single ridge waveguide is a very interesting structure, it looks similar to rectangular waveguide however with a huge capacitive loading in the middle of its broad wall. A lot of early ridged WG designs were made in the late nineteen-forties by Mr. Microwave S.B. Cohn, including some amazing filters and transitions to coaxial ports. If one is lucky enough he can see the IMS historic exhibition, I personally saw his lab-books where he analyzed such structures, and I was amazed. My luck culminated when he walked right into the exhibit in Boston 2010! I look at these works with deep respect, which is very much due for Mr. Microwave and his peers, they used their brains, no computers, no HFSS, some good math and lots and lots of intuition. Editor's note: Dr. Seymour Cohn was given the title of "Mr. Microwave" at the 1989 IMS at a special session in his honor. He passed away in September 2015. Here is his IEEE MTT-S memorial (free of charge!)
Single ridge waveguide offers very interesting set of characteristics:
- Compared to a rectangular waveguide of the same outer dimensions, ridge waveguide will have a much lower cut-off frequency of its fundamental mode. In other words for the same cut-off frequency of the fundamental mode, the cross section of the ridge waveguide will be much smaller than the rectangular waveguide which presents an opportunity for compact designs.
- By proper choice of the gap dimension (g) in relation to the b-dimension, the higher order mode can be engineered, i.e. can be pushed very far out in frequency, what is the benefit of that? This can be very useful in filter design for example , where the spurious pass-bands associated with higher order modes are pushed very far out sometimes eliminating the need for clean-up low-pass filter.
- Ridge waveguide can serve a niche area, where compactness is needed, but planar transmission lines are not able to handle some power levels, one can say it’s not as good in power handling as rectangular waveguides, but much better than microstrip lines, it’s not as good in terms of loss as rectangular waveguide but much better than microstrip lines... the argument goes very much always like this.
- With a very wide mono-mode band, the ridge waveguide can be used to a certain extent as a “true transmission line”, i.e. it can be used to realize structures that can be modeled by transmission lines, such as couplers, filters, feeds etc over very wide-bands not attainable by rectangular waveguides.
- Other uses for ridge waveguide include switches, where it’s much easier to realize a short circuit in the gap region, than it is to realize it in rectangular waveguide. A MEMS actuator or a simple screw in the gap region will provide an almost perfect short circuit, since most of the field is concentrated in that area.
Single ridge waveguide can be thought of as half of a double ridge waveguide with a horizontal perfect electrical conductor (PEC) inserted at exactly the middle of the gap region. This waveguide will have the same cut-off frequency of the fundamental waveguide mode, however the higher order modes will be pushed further away in frequency. As usual, the price paid is increased loss and reduced power handling in comparison to double ridge waveguide.
Cross section of single ridge waveguide
Some draw backs:
- Convenience: modes of the ridge waveguide cannot be found analytically, previously modal analysis was used to obtain them but with the advances in EM simulators, decent results can be obtained efficiently
- Ohmic Loss: loss is higher than the rectangular waveguide but they can be engineered and optimized
- Power handling : because of the gap region the power handling is lower than that of the rectangular waveguide but , trade-offs can be made (how far you want to push the higher order modes versus power handling versus ohmic loss), in other words ENGINEERING.
Some useful references:
- W. J. Hoefer and M. N. Burton, Closed-form expressions for the parameters of finned and ridged waveguides, IEEE Trans. Microw. Theory Tech., vol. 82, pp. 2190–2194, Dec. 1982. Here you can find formulae for calculating the cut-off frequencies, characteristics impedances etc.
- Ridge Waveguides and Passive Microwave Components, by J. Helszajn, IET Electromagnetic Waves Series 49 . A comprehensive treatment of this waveguide.